The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of UMTS (Universal Mobile Telecommunication Service) and LTE (Long Term Evolution). LTE is a technology for realizing high-speed packet-based communication that can reach high data rates in both downlink and uplink, and is thought of as a next generation mobile communication system relative to UMTS. In order to support high data rates, LTE allows for a system bandwidth of up to 20 MHz. LTE is also able to operate in different frequency bands and can operate in at least FDD (Frequency Division Duplex) and TDD (Time Division Duplex). The modulation technique or the transmission method used in LTE is known as OFDM (Orthogonal Frequency Division Multiplexing).
For the next generation mobile communications system e.g. IMT-advanced (International Mobile Telecommunications) and/or LTE-Advanced, which is an evolution of LTE, support for bandwidths of up to 100 MHz is being discussed. In both LTE and LTE-Advanced, radio base stations are known as eNodeBs or eNodeBs, where “e” stands for evolved. Furthermore, multiple antennas with precoding/beamforming technology can be used in order to provide high data rates to user equipments. Thus, LTE and LTE-Advanced both constitute examples of MIMO (Multiple-Input, Multiple-Output) radio systems. Another example of a MIMO and OFDM based system is WiMAX (Worldwide Interoperability for Microwave Access).
In LTE, inter-cell interference mitigation is a key issue to potentially improve system performance for cell-edge UEs (User Equipments). LTE technologies include some mechanisms, such as inter-cell interference coordination (ICIC), for mitigating the interference between neighboring cells. The standardized ICIC schemes for Rel-8 of LTE primarily rely on frequency domain sharing between cells and adjustment of transmit powers. Signaling used for ICIC schemes is supported in X2 interface between eNodeBs for downlink and uplink respectively.
Proactive downlink ICIC schemes are facilitated via the standardized Relative Narrowband Transmit Power (RNTP) indicator. The RNTP is an indicator per physical resource block (PRB) signaled from an eNodeB to its neighboring eNodeBs over X2 interface, which indicates the maximum anticipated downlink transmit power level per PRB. Using RNTP, it is possible to dynamically configure different re-use patterns from full frequency re-use to hard frequency re-use 110, fractional frequency re-use 120 and soft frequency re-use 130 as illustrated in FIG. 1.
One proactive uplink ICIC mechanism is standardized based on a High Interference Indicator (HII). The HII is sent from an eNodeB to its neighboring eNodeBs with a single bit per PRB over the X2 interface, indicating whether its serving cell intends to schedule cell-edge UEs causing high inter-cell interference on those PRBs, i.e. select cell-edge UEs to transmit using those PRBs. The neighboring eNodeBs should then aim at scheduling UEs with low interference at those particular PRBs to avoid scheduling of cell-edge users on the same PRBs between two neighboring cells. Use of the HII mainly provides gain for fractional frequency re-use cases. Another reactive uplink ICIC scheme is based on an Overload Indicator (OI). Low, medium and high OI reports can be signaled over the X2 interface to neighboring cells based on the measurement information.
Furthermore, in LTE-Advanced, a new technology e.g. coordinated multiple points (CoMP) transmission/reception is introduced to potentially improve ICIC performance via coordinated scheduling/beamforming (CS/CB). CoMP technology is also regarded as an enhanced ICIC technology, however additional X2 signaling would be required to support CS/CB between eNodeBs. The details thereof are being discussed in 3GPP.
As described above, there are several mechanisms of ICIC in LTE and LTE-Advanced. However some problems exist in those mechanisms, which are listed as follows.                The ICIC schemes utilizing RNTP for downlink and HII/OI for uplink need to work over X2 interface to implicitly exchange inter-cell interference information between eNodeBs. Typically, the messages sent out over X2 interface are not so frequent. That means a dynamic ICIC, such as per-TTI ICIC, is very difficult to be supported to catch up with the changes of real-time interferences.        As described in FIG. 1, hard frequency re-use 110 and fractional frequency re-use 120 can well take care of the interferences, but they are at cost of spectrum efficiency. So, an appropriate soft frequency re-use with good ICIC will be desired. As such, further investigations are needed on how to implement a good ICIC and soft frequency re-use at the same time based on the mechanisms in Rel-8.        Although CoMP targets to enhance ICIC in a dynamic way, X2 signaling is still needed in order to implement CoMP. X2 messages need be frequently exchanged between eNodeBs and accordingly a lot of scheduling/beamforming related information needs to be carried. This will result in a big load and inefficient overhead over X2 interface.        
In addition, existing ICIC mechanisms may cause greedy power selection, which greatly degrades system performance and meanwhile increases the power consumption. One example is shown in FIG. 2 that illustrates the inter-cell interferences on the uplink transmission, where two cell-edge UEs 220, 240 are served by their respective eNodeBs 210, 230. In order to maintain the proper communication between UEs 220, 240 and their respective eNodeBs 210, 230, the corresponding uplink SINR (signal to interference-and-noise ratio) at each eNodeB can be expressed as:
                                          For            ⁢                                                  ⁢            UE            ⁢                                                  ⁢            220            ⁢                          :                        ⁢                                                  ⁢                          SINR              1                                =                                                                      H                  11                                ·                                  P                  1                                                                                                  H                    21                                    ·                                      P                    2                                                  +                                  N                  1                                                      >                          SINR                              target_                ⁢                1                                                    ,                                  ⁢                              For            ⁢                                                  ⁢            UE            ⁢                                                  ⁢            240            ⁢                          :                        ⁢                                                  ⁢                          SINR              2                                =                                                                      H                  22                                ·                                  P                  2                                                                                                  H                    12                                    ·                                      P                    1                                                  +                                  N                  2                                                      >                          SINR                              target_                ⁢                2                                                                        (        1        )            
where Pi (i=1 or 2) is uplink transmit power of i-th UE, Nj (j=1 or 2) is received noise at j-th eNodeB, Hi,j denotes channel fading from i-th UE to j-th eNodeB. SINRi is measured and compared with pre-defined UE specific SINR target SINRtarget—i to judge whether to trigger adjustment of uplink power allocation. In existing methods SINRtarget is set to be a fixed value for a UE without taking into considerations UE's specific channel condition and interference situation. Specifically, the solutions of (P1,P2) that can satisfy the SINR requirements (Expression (1)) may not be exclusive, and instead multiple solutions may exist. That means vicious competition of power increase between UEs may result in a higher and higher interference, although solutions may exist to satisfy the SINR requirements for both UEs with lower transmit power and lower interference between each other. In other words, this conventional power control with fixed target SINR constraint could result in an undesirable competition of power increase among multiple UEs due to a high mutual interference. Hence, an enhanced method to tackle the problem in existing ICIC mechanisms with greedy power allocation and fixed target SINR is required to facilitate the cooperation of UE instead of competition in power selection.